Part 1. The Field of the Invention
The present invention relates to aqueous based shear-thinning ink compositions and to ball-point pens employing such ink compositions which preferably include a viscoelastic ink follower.
Part 2. Description of the Prior Art
While there are long-recognized cost advantages in employing aqueous ink compositions in ball-pen writing instruments, their typically low viscosity leads to a number of performance disadvantages. These include the possibility of leakage around the rotating ball due to changes in atmospheric pressure or temperature or due to temperature increases resulting from the heat conducted from the fingers of the user. Attempts have been made to overcome these disadvantages by employing a fibrous ink reservoir intended to supply a flow of ink to the rotating ball at a rate just sufficient to result in the formation of a continuous line during use but insufficient to provide an excess supply of liquid ink at the writing tip leading to leakage. Other attempts to accomplish the same result have involved the use of liquid ink reservoirs employing porous ink feed rods in the form of sintered fibrous bundles or extruded plastic ink rods having capillary channels and intended to control the rate of ink feed to the rotating ball. Examples of prior art patents employing such systems to control the flow of aqueous inks in ball-pen writing instruments include British Patent Specification No. 1,139,038 and U.S. Pat. Nos. 3,446,564; 3,533,708; 3,572,954; 3,873,218; and 4,145,148.
The disadvantages of using an ink-feeding system of the types discussed above include failure to provide a sufficiently continuous flow of ink to the ball for rapid use and a reduced writing life since the amount of ink retained by a fibrous reservoir may amount to as much as one-half of the total amount of ink in the writing instrument. A further disadvantage lies in the fact that inks containing pigments as opposed to soluble dyes will tend to clog the capillary passages in fibrous reservoirs and feed rods further inhibiting the rate of flow and the amount of ink which is delivered to the point before the writing instrument becomes inoperative.
If it is attempted to solve the above problems by leaving out the fibrous reservoir and/or capillary feed rod and simply thickening the aqueous ink composition, another problem is encountered. Should the continuity of the ink column above the rotating ball be broken by solvent evaporation from the point or from shock during shipping or dropping, it is likely that the pen will become non-functional.